1. Field of the Invention
The present invention relates generally to implantable, radially expandable medical prostheses which are frequently referred to as stents. In particular, the present invention is a titanium alloy self-expanding stent.
2. Description of the Related Art
Self-expanding medical prostheses frequently referred to as stents are well known and commercially available. They are, for example, disclosed generally in the Wallsten U.S. Pat. No. 4,655,771, the Wallsten et al. U.S. Pat. No. 5,061,275 and in International Application Publication Number WO 94/24961, all of which are hereby incorporated by reference in their entirety. Devices of these types are used within body vessels of humans and other animals for a variety of medical applications. Examples include intravascular stents for treating stenosis, stents for maintaining openings in the urinary, biliary, esophageal and renal tracts and vena cava filters to counter emboli.
Briefly, self-expanding stents of the type described in the above-identified patent documents are formed from a number of resilient filaments which are helically wound and interwoven in a braided configuration. The stents assume a substantially tubular form in their unloaded or expanded state when they are not subjected to external forces. When subjected to inwardly directed radial forces the stents are forced into a reduced-radius and extended-length loaded or compressed state. A delivery device which retains the stent in its compressed state is used to deliver the stent to a treatment site through vessels in the body. The flexible nature and reduced radius of the compressed stent enables it to be delivered through relatively small and curved vessels. After the stent is positioned at the treatment site the delivery device is actuated to release the stent, thereby a owing the-stent to self-expand within the body vessel. The delivery device is then detached from the stent and removed from the patient. The stent remains in the vessel at the treatment site.
Stents must exhibit a relatively high degree of biocompatibility since they are implanted in the body. Commonly used materials for the stent filaments include Elgiloy.RTM. and Phynox.RTM. spring alloys. Elgiloy.RTM. alloy is available from Carpenter Technology Corporation of Reading, Pa. Phynox.RTM. alloy is available from Metal Imphy of Imphy, France. Both of these metals are cobalt-based alloys which also include chromium, iron, nickel and molybdenum. Other materials used for self-expanding stent filaments are 316 stainless steel and MP35N alloy which are available from Carpenter Technology Corporation and Latrobe Steel Company of Latrobe, Pennsylvania, and superelastic Nitinol nickel-titanium alloy which is available from Shape Memory Applications of Santa Clara, Calif. Nitinol alloy contains about 45% titanium. Yet another self-expanding stent, available from Schneider (USA) Inc. of Minneapolis, Minn., includes an Elgiloy.RTM. alloy case with a tantalum or platinum alloy core. The tantalum or platinum alloy core is radiopaque and enhances the visibility of the stent in fluoroscopy during implantation.
The strength and modulus of elasticity of the filaments forming the stents are also important characteristics. Elgiloy.RTM., Phynox.RTM., MP35N and stainless steel are all high strength and high modulus metals. Nitinol has relatively lower strength and modulus.
There remains a continuing need for self-expanding stents with particular characteristics for use in various medical indications. Stents are needed for implantation in an ever growing list of vessels in the body. Different physiological environments are encountered and it is recognized that there is no universally acceptable set of stent characteristics. In particular, there is a need for stents formed from moderate strength materials having lower moduli of elasticity than those of Elgiloy.RTM., Phynox.RTM., MP35N, and stainless steel from which certain stents are currently formed. Stents formed from moderate strength and relatively low moduli of elasticity materials would have properties adapted to an expanded range of treatment applications. Stents with lower moduli of elasticity material would be less stiff and more flexible than a stent made of the same size wire and same design utilizing a high modulus material. Stents of these types must also exhibit a high degree of biocompatibility. Furthermore, the filaments from which the stent is fabricated are preferably radiopaque to facilitate their implantation into patients.
The current self-expanding stents made of Elgiloy.RTM., MP35N, stainless steel, and nitinol can be made to have various characteristics by varying filament wire sizes and stent designs. However, a group of stent wire materials with properties between those of very high strength, high modulus materials (Elgiloy.RTM., MP35N, stainless steel) and of low strength, low modulus materials (NITINOL) would allow even more stent variants to be produced.
The implantation of an intraluminal stent will preferably cause a generally reduced amount of acute and chronic trauma to the luminal wall while preforming its function. A stent that applies a gentle radial force against the wall and that is compliant and flexible with lumen movements is preferred for use in diseased, weakened, or brittle lumens. The stent will preferably be capable of withstanding radially occlusive pressure from tumors, plaque, and luminal recoil and remodeling.